An international team of astronomers, including researchers from the University of Cambridge, used data gathered by the Kepler Space Telescope to observe and confirm details of the outermost of seven exoplanets orbiting the star TRAPPIST-1.

The observations confirm, as had been predicted, that the seventh and outermost planet, TRAPPIST-1h, orbits its star every 18.77 days. The results were reported in the journal Nature Astronomy.

“TRAPPIST-1h was exactly where our team predicted it to be,” said Rodrigo Luger, a PhD student at the University of Washington and the paper’s lead author. The researchers discovered a mathematical pattern in the orbital periods of the inner six planets, which was strongly suggestive of an 18.77 day period for planet h.

TRAPPIST-1A is a middle-aged, ultra-cool dwarf star, much less luminous than the Sun and only a bit larger than Jupiter. The star, which is nearly 40 light years away in the constellation of Aquarius, is named after the ground-based Transiting Planets and Planetesimals Small Telescope (TRAPPIST), the facility that first found evidence of planets around it in 2015.

Artist’s impression of TRAPPIST-1. Credit: IoA/Amanda Smith

The TRAPPIST survey was led by Michaël Gillon of the University of Liège, Belgium, who was also a co-author on this research. In 2016, Gillon’s team announced the detection of three planets orbiting TRAPPIST-1 and this number was upped to seven in a paper published earlier this year. All seven planets are deemed temperate, meaning that under certain geologic and atmospheric conditions, water could exist in a liquid form. Three of the planets are particularly optimal. In addition the TRAPPIST-1 system is currently the most convenient to remotely explore the atmospheres of planets with sizes similar to Earth.

Such exoplanets are detected when they transit, or pass in front of, their host star, blocking a measurable portion of the light. “We only captured one transit of TRAPPIST-1h last autumn. However, the resonant pattern formed by the other six planets, and the time TRAPPIST-1h takes to pass in front of its star, allowed the team to deduce its orbital period with a precision of a few minutes,” said co-author Amaury Triaud, a Kavli Exoplanet fellow Amaury Triaud at Cambridge’s Institute of Astronomy. “This is absolutely remarkable! TRAPPIST-1h represents a perfect illustration of the power of the scientific method, of its ability to make predictions that can later be verified.”

The inner six planets occupy orbits consistent with being in ‘resonance’. All orbital periods are mathematically related and slightly influence each other. Orbital resonances can also be found in our solar system. For instance, Jupiter’s moons Io, Europa and Ganymede are set in a 1:2:4 resonance, meaning that while Ganymede orbits Jupiter once, Europa does so twice, and Io four times. The prediction of TRAPPIST-1h’s orbital period principally relied on extrapolating the known resonant configuration of the inner six planets, to the seventh. This prediction was later confirmed.

The team analysed 79 days of observation data from K2, the second mission of the Kepler Space Telescope, and was able to recover four transits of TRAPPIST-1h across its star. The K2 data was also used to further characterize the orbits of the other six planets, help rule out the presence of additional transiting planets, and learn the rotation period and activity level of the star.

TRAPPIST-1’s seven-planet chain of resonances establishes a record among known planetary systems. The resonances strengthen the long-term stability of the planetary system. It is also likely that these orbital connections were forged early in the life of the TRAPPIST-1 system, when the planets and their orbits were not fully formed.

“Observing TRAPPIST-1 with K2 was an ambitious task,” said Marko Sestovic, a PhD student at the University of Bern and second author of the study. In addition to the complicated signals introduced by the spacecraft’s wobble, the faintness of the star in the optical (the range of wavelengths where K2 observes) placed TRAPPIST-1h “near the limit of what we could detect with K2,” he said. To make matters worse, Sestovic said, one transit of the planet coincided with a transit of TRAPPIST-1b, and one happened during a stellar flare, adding to the difficulty of the observation. “Finding the planet was really encouraging,” Luger said, “since it showed we can still do high-quality science with Kepler despite significant instrumental challenges.”